1. Field of the Invention
This invention relates to electrostatic precipitators. More particularly this invention relates to an apparatus for stabilizing the grid frame used to keep the charged wires of an electrostatic precipitator separated from each and from swaying.
2. Background of the Invention
Electrostatic precipitation is a well known method of removing particles from polluted exhaust gas produced from, for example, industrial processes or by oil-fired electric power generation. A simple electrostatic precipitator consists of a grounded tube through which exhaust gas passes, and a wire located in the center of the tube. The placement of a large voltage between the discharge wires and grounded walls of the precipitator causes a corona discharge to occur. Particles in the exhaust gas are ionized in the corona discharge and are moved by the electric field to the grounded walls of the container where they collect for subsequent removal. Because the particles are removed from the gas stream, the exhaust ultimately released into the air has a lower solids content. In order for this procedure to be effective and efficient, it is important to maintain the greatest potential differential possible between the wire (the high-potential component) and the grounded tube.
Large precipitators can be several stories high. They require the use of very high potentials--on the order of 50,000 volts or more--and very long charged wires so as to increase particle-removing effectiveness. Usually these precipitators include a plurality of discharge wires placed in a frame, with a plurality of grounded collector electrodes spaced between the wires. In an electrostatic precipitator such as this, the ionized particles are attracted to the collector electrodes where they collect. The collector electrodes are then rapped to shake the collected particles down into a collection hopper. The particles, ash, and any other solid contaminants, are then withdrawn from the hopper.
Electrostatic precipitators, particularly the larger ones as briefly described above, suffer from a problem caused by the electrostatic field surrounding the discharge wires. This field causes the discharge wires to oscillate. As the wires oscillate, they swing toward the collector electrodes--often getting close enough to cause electric arcing. This arcing causes several problems. First, the arcing greatly reduces the electrostatic field strength surrounding the discharge wires, thus reducing the efficiency of the precipitator. Second, the arcing causes pitting--similar to the deterioration observed on electric switch contacts--of the collector surfaces and discharge wires by physically removing pieces of those components.
One way to reduce oscillation is to attach weights or "bottles" to the bottom ends of the discharge wires. However, in large precipitators weights are ineffective because the electric fields in these precipitators are so strong that oscillation occurs even when the wires are weighted. To overcome this problem weight guide grid systems or frames were developed for large precipitators. A grid system is suspended below the collector electrodes from a top frame and thus is isolated from the grounded shell structure of the precipitator. The grid system is arranged so that weights from the discharge wires are suspended within the system. The grid reduces, but does not eliminate discharge-wire oscillations. Further, the oscillations of large, high-voltage wires cause the entire grid or frame itself to oscillate or "swing." The frame swing in large precipitators causes the same arcing problems that occur in smaller precipitators due to discharge-wire oscillation.
In an attempt to reduce frame swing, grid frames have been anchored to the wall of the electrostatic-precipitator hopper or other shell structure. However, since the grid, having the same potential as the bottom of the wires, is at a different potential than the grounded hopper or other anchoring points, insulators must be used to anchor the grid. Otherwise, a current path is created between ground and the discharge wires. This can cause a catastrophic short or, at a minimum, it can cause a tremendous loss of electric potential between the discharge wires and the collector electrodes, thus limiting the ability of the precipitator to draw particulate out of the gas stream. Prior-art attempts at anchoring or stabilizing grid systems have proven unsatisfactory because of particle-accumulation-caused shorting currents.
One such prior-art attempt at providing an electrostatic precipitator stabilizer is described in U.S. Pat. No. 3,972,701 issued to Teel. Teel discloses a rigid insulator-stabilizer means connected between the collector electrodes and the weight-guide grid. The stabilizer includes a single, well-known type of rigid ceramic electrical insulator coupled between a top support assembly and a bottom support assembly. The top support assembly is connected to a pair of fixed bottom beams which are connected to the collector electrodes. The top support assembly also includes a sleeve to receive the top portion of the insulator. The bottom support is connected to the grid frame (which is connected to the electrode wires) and includes a cylindrical cup to receive the bottom portion of the rigid insulator. The stabilizer permits the insulator to slide vertically but limits its lateral motion. In effect, the Teel stabilizer prevents grid swing by connecting the grid frame to the fixed bottom beams of the low-potential collector electrodes through the insulating stabilizer.
However, the noted system has at least one significant shortcoming. It places the insulating stabilizers in the direct path of particles falling from the collector electrodes down to the collection hopper. This means that particles will build up on the surface of the stabilizer and thereby create a conducting bridge between the grid frame and the fixed bottom beams. Of course, this bridge of conductive particles can be a path for current. Thus, in the Teel device, a particle layer will inevitably build up creating a path between the high-potential grid frame and the low-potential fixed bottom beams. Current through this particle layer causes a tremendous reduction in the voltage differential between the collector electrodes and the discharge wires and thereby reduces the effectiveness as well as the energy efficiency of the precipitator. This is a common occurrence in electrostatic precipitators utilizing stabilizers in the path of the falling particles.
Teel teaches the use of an unglazed insulator to counteract the effects of particle accumulation on the surface of the insulator. However, it is likely that even the surface of an unglazed insulator will, over time, be oxidized due to transient currents in the particles that accumulate on the insulator. When oxidized, the insulator can become conductive and it thereby creates a permanent current path--until the insulator is replaced--between the grid frame and the collector electrodes. Further, Teel does not address the problem of particle accumulation on other parts of the stabilizer, which also may cause shorting currents between the grid frame and the collector electrodes. Of course, particle accumulation can be addressed by physical cleaning; however this a costly and time-consuming chore that makes maintenance more burdensome. Furthermore, it is noted that for large precipitators it is extremely difficult to access the discharge area. Often, the scheduled cleaning required to maintain a relatively efficient precipitator can take one to two days--a considerable shutdown time for industrial operations.
Present stabilizers have another deficiency. Stabilizers are subject to stress and, over time, break. Unfortunately, when they break, stabilizers such as the one disclosed by Teel, fall directly into the ash hopper, thereby clogging it. This requires emergency, rather than scheduled, maintenance procedures.
In response to these problems, particularly the problem of particle accumulation, there have been attempts to move stabilizers out of the particle stream. Those attempts involved the use of relatively long support bars to attach the stabilizers to the wire frames. However, such fixes are unsatisfactory because the long support bars used have little lateral stability and hence will not effectively eliminate grid-frame swing. Further, the long support bars are extremely difficult to access, since they are usually far removed from the falling particle stream and are thus well away from the access areas used to work on the grid frame and the ash hopper. In addition to the failure to prevent frame swing and the failure to ease maintenance difficulties, these and other similar stabilizer "fixes" are very expensive to produce.
Hence, what is needed is a grid-frame stabilizer that eliminates grid-frame swing so that arcing between the discharge wires and collector electrodes is prevented. Further what is needed is a stabilizer that is not susceptible to particle build-up so that the creation of shorting, particle-layer-current paths between the discharge wires and ground is prevented. Still further, what is needed is a grid-frame stabilizer that significantly reduces maintenance requirements and that is relatively inexpensive with respect to other similar devices used to reduce the problems associated with frame swing in electrostatic precipitators.